There are special electrolytes supposedly geared for commercial poultry. The ones I saw came in a huge packet for a 20-gallon water can, so I divided it up and mixed it with regular water one gallon at a time. Basically it is supposed to be like gatorade for chickens, but I don't think it does much good. I only use it a couple days each year, when the temperature is going to be up above 100.

A simpler way to keep them cool (and one they certainly seem to enjoy more) is to make sure they have plenty of shady dust to burrow into. I run a sprinkler over the area the evening before, so it has plenty of time to get even cooler over night (but not muddy). They'll burrow down into that to get to the cooler dirt underneath: basic dust bath stuff. Another thing I do is put a sprinkler onto a small patch of grass in the very late afternoon, and they seem to enjoy bopping around in the wet grass before bed as well. I have no idea if it helps, but if they like it, I don't see that it can hurt anything.

Birds were staying cool before hominids walked upright, and if they are not confined into an artificial space, they seem to do a good job of finding what they need: shade, water, dustbaths, etcetera.